Loudspeaker systems comprise a plurality of radiating devices for the conversion of electrical signals into acoustic energy (sound). There exists at least two basic types of radiating devices, those of the horn-type and those that have no horn (direct radiator). Each radiating device comprises a driver or motor for the conversion of electrical to mechanical energy. The driver is attached to a diaphragm which distributes mechanical energy of the driver to the air. In a horn-type device, a horn (a rigid, non-absorbing, tapered duct) directs the acoustic energy from the driver and diaphragm to the open end or mouth of the horn. In a hornless driver, the diaphragm is supported by a diaphragm housing which serves to space the driver from the annulus or mouth of the diaphragm.
It is desirable for the sound from each radiating device in a speaker system to arrive at the listening location simultaneously. It is further desired that acoustic wavefronts propagating from each radiating device diverge from a single plane or surface. In a typical speaker system, placement of the radiating devices to satisfy the first requirement (simultaneous arrival of acoustic energy) makes satisfaction of the second requirement (divergence of sound from a single plane or surface) not possible.
The importance of satisfying these two criteria cannot be understated. It is of special importance for loudspeaker systems used in auditoriums. The ability of the audience to hear and understand an individual using the loudspeaker system may be dramatically increased by satisfying these two requirements. Even though the high and low frequency portions of words and syllables may reach the listener only split seconds apart, the ability of the listener to process the information received will be greatly increased by arranging for the simultaneous arrival of the high and low frequency portions.
In prior art speaker systems, this problem has been addressed by placing the radiating exits of the acoustic devices in a single plane or surface. (The radiating exit of a horn-type radiating device is the mouth of the horn. The radiating exit of a direct radiator device is the annulus of the diaphragm.) An electronic delay is applied to the input signal of any radiating device whose effective length is less than that of the radiating device with the longest effective length. By effective length is meant the distance from the driver to the radiating exit. The acoustic energy from each radiating device is thereby made to arrive at the listening location simultaneously with that from all other devices.
FIG. 1 illustrates an arrangement in accordance with the prior art. A high frequency horn 10 is mounted with the mouth thereof at the panel 11. A low frequency direct radiator is mounted with the annulus 12 of the diaphragm 13 at the panel. A diaphragm housing 14 supports the diaphragm as shown. The effective length of the horn is the distance L2 between the driver 15 and the panel. The effective length of the direct radiator is the distance L1 between the driver 16 and the panel. Due to the differences in the effective lengths L1 and L2 of the two devices, any electrical signal applied simultaneously to both radiators will result in high frequency energy arriving in advance of low frequency energy. The signals are synchronized by applying an electronic delay .tau. to the high frequency signal prior to the driver 15. .tau. is determined by the following formula: EQU .tau.=(L1-L2)/s
where s is the speed of sound in air. The delay allows all acoustic energy to arrive simultaneously at the listening position.
The arrangement of FIG. 1 has several drawbacks. All available electronic delay devices 17 must operate at low signal levels (i.e., the signal chain prior to power amplification). This means that one power amplifier 18, 19 is required for each different delay. Bandwidth requirements will often dictate the use of complex and expensive electronic delay systems. The added complexity adds to the cost and the likelihood of component failure. Since the velocity of sound in air varies with temperature, the delay time chosen cannot be correct for all conditions under which the speaker system may be called to perform.